1. Field of the Invention
The invention relates to a fuel cell system incorporating a steam generation system that has an evaporating portion for generating steam for fuel reforming from raw material water (water as a raw material), and relates to a fuel cell system diagnosis method.
2. Description of Related Art
Japanese Patent Application Publication No. 2008-243594 describes a technique of detecting generation of steam by feeding raw material water to an evaporating portion in a fuel cell system. According to this technique, generation of steam is detected based on a change in the temperature detected by a temperature sensor that is provided near a raw material water inlet of the evaporating portion. As steam is generated from the raw material water, the temperature of the ambient atmosphere decreases due to evaporation heat, and therefore generation of steam is detected based on such a decrease in the temperature of the ambient atmosphere. Further, in the fuel cell system described in Japanese Patent Application Publication No. 2008-159466, the feeding of raw material water is detected through a comparison between the control signals for a raw material water feed pump and the signals of a flow sensor that is provided midway in a water pipe.
According to the technique described in Japanese Patent Application Publication No. 2008-243594, if the evaporation heat of the raw material water is at a certain level or higher with respect to the thermal capacity of the evaporating portion, the temperature decreases as a result of generation of steam, and therefore the generation of steam can be detected. However, if only a small amount of the raw material water is fed and therefore the level of the evaporation heat is low, the generated steam causes only a small change in the temperature. In such a case, the generation of steam may not be detected properly. According to the technique described in Japanese Patent Application Publication No. 2008-159466, further, although it can be detected whether the raw material water in a liquid phase is fed to the evaporating portion, it is impossible to directly determine whether steam is being actually generated or has been actually generated. Thus, the technique described in Japanese Patent Application Publication No. 2008-159466 is unable to cope with possible abnormalities, such as the raw material water leaking out from a midway point of the pipe and an evaporation failure due to a local temperature decrease.
Accordingly, it is not easy to determine whether raw material water has been fed to an evaporating portion and steam has been generated. Thus, there is a possibility of a lack of steam at the reforming portion. If the fuel is fed to the reforming portion despite that the steam is not sufficient, coking may occur at the reforming portion, which is not desirable in terms of the life of the reforming portion, that is, the life of the fuel cell system. It is to be noted that “coking” represents that carbon derived from the fuel fed to the reforming portion deposits on the surface of the reforming catalyst in the reforming portion, and it results in a decrease in the life of the reforming catalyst.